accfft_local_size calls and integer overflow

[jrt54]

When looking at the "step2f" example and using a domain of size 840 x 840 x 840, I get a segfault. This is on a 64-bit CPU architecture.

I think the culprit is the routine "accfft_local_size_dft_r2cf", which returns an integer and encounters overflow. The returned value was 0 to "alloc_max", which then results in a segfault due to subsequent malloc calls using a value of 0.

Note that if I use 2 ranks, this no longer results in a segfault, since I just barely avoid issues with overflow.

Can accfft_local_size_dft_r2cf be changed to return a size_t type or unsigned int instead?